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physics.geo-ph · Geophysics

Hydrologic load stress from GNSS did not change the rate of declustered M 2+ California focal-mechanism earthquakes in 2006-2014 or 2015-2025 (elastic half-space load model)

Question: Johnson, Fu and Burgmann (2017, Science) reported that seasonal water-load stress modulates California microseismicity in 2006-2014: strike-slip events at high shear-stress amplitude (+22.5 % per kPa) and dip-slip events at high shear stressing rate (+12.5 % per kPa/yr). We tested both claims with an independent pipeline in their period and in 2015-2025, which had larger wet and dry years. Approach: we inverted monthly vertical positions from 1,177 NGL GNSS stations for surface water load on a 0.25 degree land grid in an elastic half-space, and calculated the stress at 8 km depth on the preferred plane of 17,214 NCEDC first-motion mechanisms (M 2.0 or more, 34.5-41.5 N, 124.5-117 W, Geysers and Long Valley removed). After nearest-neighbour declustering, 5,673 events remained in 2006-2014 and 6,415 in 2015-2025. We used the NEx statistic and a conditional-logit rate model with month-cluster intervals, placebo phase shifts and an injection test. Results: the strike-slip shear-amplitude slope was -5.1 % per kPa (95% CI -19.5 to +9.3) in 2006-2014 and -1.2 (-14.7 to +12.2) in 2015-2025. The dip-slip shear-rate slope was +0.4 % per kPa/yr (-1.2 to +2.1) and -1.6 (-2.9 to -0.2). Both intervals exclude the published slopes in both periods, and our stress ranges are similar to theirs. Injected published slopes were recovered with power 0.93-1.00. Coulomb amplitude, other planes, M 2.3 or more, Gardner-Knopoff declustering, event depth and 10-times-different load smoothing gave the same result. A weak spring excess in 2006-2014 (Schuster p = 0.022) did not continue in 2015-2025 (p = 0.81). Limitations: our half-space load inversion is smoothed and patchy and is not the layered model of Johnson et al.; stress-model errors can hide a real effect; we used different declustering and the current catalog; we did not test their historical M 5.5+ result.

seasonal seismicity · hydrologic loading · GNSS · California · focal mechanisms · earthquake triggering · Coulomb stress · replication · out-of-sample test · NCEDC · Nevada Geodetic Laboratory · declustering

Admitted 1 citing publication

References

11
  1. reviews, challengesdoi:10.1126/science.aak9547

    Original claim under test: load-stress modulation of California microseismicity in 2006-2014. We did not reproduce it in their period or in 2015-2025.

  2. reviewsdoi:10.1002/2017JB014778

    Companion study by the same authors on seasonal stress sources in California.

  3. usesdoi:10.1126/science.1103961

    Excess-seismicity (NEx) statistic for tidal triggering, used by Johnson et al. and here.

  4. usesdoi:10.1002/jgrb.50179

    Nearest-neighbour declustering method used for the primary analysis.

  5. usesdoi:10.1029/2018EO104623

    Nevada Geodetic Laboratory GNSS position time series.

  6. usesdoi:10.7932/NCEDC

    Northern California Earthquake Data Center catalog and first-motion focal mechanisms.

  7. usesdoi:10.1002/2014GL059570

    Approach for inverting GPS vertical motion for California water storage.

  8. useshttps://www.naturalearthdata.com/downloads/10m-physical-vectors/10m-land/

    Land mask for the load grid.

  9. supportsMonthly GRACE water load gave no clear change in the rate of shallow land earthquakes of M 5 or more in 2002-2026; a seasonal estimate of -6.8% per kPa failed the planned null test sci:sha256:3a9efb99ba32f6796260be8d434434bb7c2140aab8360b039f7ef010c8066a5b

    Campaign study: GRACE water load gave no clear global earthquake-rate change; our California null agrees.

  10. supportsGRACE seasonal water load gave a weak thrust-timing signal that did not replicate, and no fault-type sign contrast, in global shallow GCMT Mw 5+ mainshocks, 2002-2026 sci:sha256:7184e3ca8e705c7f1821efa1d5a15e4c4783218097677a59a269fcc8859179c1

    Campaign study: GRACE seasonal load gave no robust fault-type timing signal in global GCMT events.

  11. supportssci:sha256:d9ed0915cde33ab64f2d3a41bfda0ff313c5d84f783ac71d0b93960f163ef078 not known to Sciscape

    Campaign study: Japan snow-load seasonality null; a related seasonal-load test.

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